1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and more specifically to a control of electric current supplied to a heater in a fusing device that heats and fixes a toner image carried on a recording medium.
2. Description of the Related Art
An image forming apparatus using an electrophotographic process has been known. In this image forming apparatus, an unfixed image (toner image) formed on a recording medium (print paper) by an image forming means such as the electrophotographic process is fixed on the paper by a fusing device. Among known fusing devices are a heat roller type fusing device using a halogen heater and a film heating type fusing device using a ceramic planar heater as a heat source, disclosed, for example, in Japanese Patent Application Laid-open Nos. 63-313182(1988), 2-157878(1990), 4-44075(1992), 4-44076(1992), 4-44077(1992), 4-44078(1992), 4-44079(1992), 4-44080(1992), 4-44081(1992), 4-44082(1992), 4-44083(1992), 4-204980(1992), 4-204981(1992), 4-204982(1992), 4-204983(1992) and 4-204984(1992).
Generally electric power is supplied from an ac power source through a switching device such as triac to these heaters.
In a fusing device using a halogen heater as a heat source, a temperature of the fusing device is detected by a temperature detecting element such as thermistor heat sensing element. Based on the detected temperature, an on/off operation of the switching element is controlled by a sequence controller, i.e., the power supply to the halogen heater is on/off-controlled so that the temperature of the fusing device is kept at a target temperature.
In a fusing device using a ceramic planar heater as a heat source, the sequence controller determines a phase angle or wave number corresponding to a calculated power ratio supplied to the ceramic planar heater according to a difference between the temperature detected by the temperature detecting element and the predetermined target temperature. Based on the phase or wave number thus determined, the switching element is on/off-controlled for the temperature control of the fusing device.
The fusing device of heat roller fixing type basically comprises a heat roller in the form of a heating roller (fixing roller) and an elastic pressure roller in the form of a pressing roller brought into pressure contact with the heat roller. In the heat roller fixing type fusing device, the pair of rollers are rotated to introduce between their pressure nip portions (fixing nip potions) a recording medium (such as image transfer sheet, electrostatic recording paper, electrofax paper and printing paper) which caries an unfixed image (toner image) to be fused, so that the recording medium is held under pressure between and fed by the two rollers. In this process, the heat roller type fusing device permanently fixes the unfixed image onto the recording medium (referred to as a transfer material) by the heat from the heat roller and the pressure of the pressure nip portions.
The film heating type fusing device (on-demand fusing device) is proposed, for example in Japanese Patent Application Laid-open Nos. 63-313182(1988), 2-157878(1990), 4-44075(1992) and 4-204980(1992). In these on-demand fixing devices, a heat resisting film (fixing film) as a heating roller is held against a heating body with a pressure roller (elastic roller) for sliding transport. Next, a transfer material carrying an unfixed image is introduced, along with the heat resistant fixing film, into a pressure nip portion formed by the heating body and the pressure roller and fed through the nip portion. As a result, the unfixed toner image is fixed on the transfer material as a permanent image by the heat from the heating body and the pressure from the nip portion. applied through the heat resistant film.
The film heating type fusing device can use a linear heating body with a low heat capacity and a thin film with a low heat capacity Therefore, this type of fusing device can reduce power consumption and wait time (quick start capability is assured). Further, the film heating type fusing device is known to drive the film by a drive roller provided on an inner side of the film or by a frictional force with the pressure roller used as the drive roller. However, in recent years the pressure roller drive method, which uses a smaller number of parts and is less expensive, is often used.
A known current detection circuit for detecting an electric current supplied to the heater of the fusing device is shown in FIG. 1 (Japanese Patent Application Laid-open No. 5-281864(1993)). This current detection circuit has a current transformer T1, a bridge diode D1, a capacitor C1, a resistor R1 and a voltmeter.
An ac power supply P1 is smoothed by a bridge diode D2 and a capacitor C2 and connected to a low voltage power supply, The current transformer T1 is connected to a line connected to the bridge diode D2 via a resistor R2.
When a current flows through the current transformer T1, a voltage of a proportional magnitude develops across a winding on a side opposite the power line (on a secondary side). The induced voltage is smoothed by the bridge diode D1 and the capacitor C1 and a terminal voltage of the resistor R1, i.e., a voltage proportional to the input current, is detected.
As to the control of a current supplied to the heater of the fusing device, however, there are the following problems.
A first problem is that the ac power to be supplied to the ceramic planar heater has a wide voltage range of, for example, 85V–140V or 187V–264V. Hence, the power supplied to the ceramic planar heater at a full duty has a wide variation such that the power supplied at the maximum voltage of the 85–140V voltage range is about 2.7 times that supplied at the minimum voltage of the same range. Also, the same supplied power has a wide variation such that the power supplied at the maximum voltage of the 187–264V voltage range is about 2 times that supplied at the minimum voltage of the same range.
Further, the current supplied to the ceramic planar heater is controlled by the sequence controller so that a predetermined temperature is kept. Thus, as the thickness of paper to be passed through the fusing device increases, the power or current that needs to be supplied increases. Depending on the kind of paper, more power than is necessary is supplied to the ceramic planar heater.
A second problem is that the fixing capability of a toner image on the transfer material in the fusing device is known to be influenced greatly by the thickness and surface roughness of the transfer material. Paper with a rough surface in particular has a significantly degraded fixing performance.
This is caused by the fact that a reduced contact area between the heating member and the paper in the nip portion results in a sufficient amount of heat failing to be supplied to the toner on the transfer material.
To obtain a good fixing performance even with a paper kind with rough surface, it is therefore necessary to increase the fixing pressure and the fixing temperature. However, increasing the fixing pressure tends to increase a drive torque of the fusing device and therefore the device cost. On the other hand, simply increasing the fixing temperature to obtain an improved fixing performance can result in an excessive amount of heat being supplied to thin paper and paper with good surface. This in turn causes problems such as hot offsets and increased curling of paper.
Optimum fixing requirements for both kinds of paper with rough surface and with smooth surface are difficult to satisfy and the conventional practice involves selecting an appropriate fixing temperature setting according to the kind of paper on the part of the user. However, setting the fixing mode using the surface roughness, a parameter that the user cannot easily understand, is not easy and there has been a call for a capability of automatically performing an appropriate fixing temperature setting according to the kind of paper.
A third problem is that since an output voltage of the current transformer T1 is full-wave rectified, it is very difficult to detect a current particularly when a phase control, which is often performed during a power control in an image forming apparatus, is executed.
Therefore, the control of current supplied to the heater in the fusing device may become inaccurate.